Add RemoteEstimatorProxy for capturing receive times

For use when send-side bandwidth estimation is enabled.

Receive times need to be captured, buffered and then sent using
TransportFeedback RTCP messaged back to the send side.

BUG=webrtc:4173

Review URL: https://codereview.webrtc.org/1290813008

Cr-Commit-Position: refs/heads/master@{#9898}
diff --git a/webrtc/modules/modules.gyp b/webrtc/modules/modules.gyp
index a195683..a27516d 100644
--- a/webrtc/modules/modules.gyp
+++ b/webrtc/modules/modules.gyp
@@ -223,6 +223,7 @@
             'remote_bitrate_estimator/remote_bitrate_estimator_single_stream_unittest.cc',
             'remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.cc',
             'remote_bitrate_estimator/remote_bitrate_estimator_unittest_helper.h',
+            'remote_bitrate_estimator/remote_estimator_proxy_unittest.cc',
             'remote_bitrate_estimator/send_time_history_unittest.cc',
             'remote_bitrate_estimator/test/bwe_test_framework_unittest.cc',
             'remote_bitrate_estimator/test/bwe_unittest.cc',